This invention relates to data storage systems. More particularly, this invention relates to a method and system for reducing the noise encountered when providing positioning values to an access device to position the access device over a data storage medium.
Conventional sector servo direct access storage devices (xe2x80x9cDASDsxe2x80x9d) have a fixed number of servo sectors (N) with the disks spinning at a fixed rotation speed (w revolutions per second). The sampling rate (or frequency) fs corresponds to N*w samples per second. The servo bursts in the servo sectors are referred to herein as a stream of positioning bursts, and the bursts of this stream are therefore sensed by a sensing device at the rate at which they move past the sensing device, or fs. Digital servo controller designs are usually implemented assuming this sampling rate.
Seek acoustic noise generated by a DASD mechanism is a significant problem. The acoustic noise generated during seek arises from broadband forcing of the environmental components or from the excitation of xe2x80x9cpure tonexe2x80x9d modes at a well defined frequency fs. It is critical to minimize seek acoustics without compromising the access performance of a DASD. Pure tone modes due to structural resonance can get amplified through the servo feedback process or due to roughness (significant step change) in the digital-to-analog converter (xe2x80x9cDACxe2x80x9d) output value.
The configuration of a typical DASD servo is shown in FIG. 1. An exemplary DASD 10 is depicted having a sensing device (actuator 14 coupled to read/write electronics 12). It should be understood to those skilled in the art that the terms xe2x80x9csensing devicexe2x80x9d and xe2x80x9caccess devicexe2x80x9d used herein should be construed broadly and without limitation to connote parts of the same subsystem within the DASD, or components of different subsystems. An access device (actuator 14, voice coil motor (xe2x80x9cVCMxe2x80x9d) 15, and spindle/VCM Driver 22) is also depicted.
A position error signal (xe2x80x9cPESxe2x80x9d) 32 (derived from a sensed position burst via line 30) from peak detecting demodulator 16 is initially sampled by an analog-to-digital converter 26 within a microprocessor unit (xe2x80x9cMPUxe2x80x9d) 18 and sent to the hard disk controller (xe2x80x9cHDCxe2x80x9d) 20 for PES non-linearity processing 24, where known distortion of the PES is first corrected (by the HDC without burdening the MPU). The linearized PES is sent back to the MPU for conventional seek servo computation in Seek/Settle/Tk-Follow subsystem 28. During seek, a velocity servo technique is used to produce an output positioning control value (referred to herein simply as a positioning value) denoted by Un where n is the sampling instant. For every sector denoted by nxe2x88x921, n, n+1 . . . etc., a new control output or positioning value is computed (possibly provided by a DAC, and typically with some time delay) and sent to the access device via line 34. The positioning values 34 are therefore calculated as a function of the sensed positioning (servo) bursts stored on the storage medium.
What is required is a method and system which reduce the noise associated with the fixed frequency sampling rates of conventional servo systems, such as the one depicted in FIG. 1. The servo computation complexity should be minimized to maintain the use of low cost MPUs in DASD designs.
The shortcomings of the conventional approaches are overcome by the present invention which, in one aspect, is a method and system for positioning an access device used to access data on a data storage medium. The data storage medium has a stream of positioning bursts stored thereon. The data storage medium is moved relative to a sensing device such that at least some sequential positioning bursts of the stream of positioning bursts move past the sensing device at a first frequency. The at least some sequential positioning bursts are sensed with the sensing device. The access device is positioned using a stream of positioning values generated at a second frequency which is greater than the first frequency. One or more of the positioning values of the stream of positioning values are calculated as a function of respective bursts of the sequential positioning bursts.
For a first positioning burst of the sequential positioning bursts, a desired positioning value is generated relative to a present positioning value. An intermediate positioning value is also generated which has an amplitude between the present positioning value and the desired positioning value. The generated stream of positioning values (at the second frequency) therefore includes the desired positioning value and the intermediate positioning value. The device is positioned using the intermediate positioning value and the desired positioning value. The amplitude of the intermediate positioning value may be halfway between the present positioning value and the desired positioning value.
In another aspect, the present invention is a method and system for positioning an access device used to the access data on the data storage medium. Again, the data storage medium has a stream of positioning bursts stored thereon. The data storage medium is moved relative to a sensing device, and a first positioning burst of the stream of positioning bursts is sensed with the sensing device. The access device is positioned by asserting at least one positioning value beginning at at least one randomly calculated time point following the sensing of the first positioning burst. The at least one positioning value is calculated as a function of the first positioning burst.
A desired positioning value may be generated relative to a present positioning value, and an intermediate positioning value may also be generated having an amplitude between the present positioning value and the desired positioning value. In this case, the at least one positioning value includes the desired positioning value and the intermediate positioning value. The access device is positioned by asserting the intermediate positioning value at a first randomly calculated time point following the sensing of the first positioning burst, and the access device is then positioned by asserting the desired positioning value beginning at a second randomly calculated time.
The sequential positioning bursts may be sensed at a first frequency determined by the rate at which the medium is moved past the sensing device. In this case, the respective first randomly calculated time points and the respective second randomly calculated time points are each calculated as either a random advance or a random delay from respective time points occurring at a second frequency, the second frequency being greater than the first frequency.
According to the oversampling, randomized sampling, or randomized oversampling principles of the present invention, seek acoustic noise and pure tone build-up are minimized when positioning an access device used to access a data storage medium.